An organic electroluminescent element has a simple element configuration and can emit light in various colors, and recently, therefore, has become developed actively as a technique for producing light-emitting devices such as displays, illuminations, etc.
In the organic electroluminescent element, holes and electrons are injected from the anode and the cathode to thereby make the charges reach the light-emitting layer, and in the light-emitting layer, the charges are recombined to emit light. From this theory, for example, it is investigated to improve the luminescent efficiency by keeping the charges staying in the light-emitting layer (see PTL 1).
On the other hand, however, keeping the charges staying in the light-emitting layer worsens the current-voltage characteristics of the organic electroluminescent element. For keeping charges in one layer, in general, there is employed a method of keeping the charges therein by providing a charge trap level inside the film. According to the method, it is possible to increase the luminescent efficiency by keeping the charges in the light-emitting layer, which, however, at a time results in worsening the current-voltage characteristics. For example, “Organic EL Technology and Material Development (CMC Publishing)” says that a light-emitting material acts as a charge trap for a charge transport material therefore resulting in voltage increase (see NPL 1). In addition, “Proc. of SPIE Vil 4800, 164-171 (2003)” reports that, when 1-NaphDATA that is a charge transport material is added to αNPD that is also a charge transport material, then the former causes a charge trap to result in voltage increase (see NPL 2).
From the above, it is desired to further improve the current-voltage characteristics for putting an organic electroluminescent element into practical use as a light-emitting device.